As described in U.S. Pat. No. 5,073,134 a standard boat drive has a housing, a tube shaft rotatable in the housing about a shaft axis and having axially opposite front and rear ends, a hub carried on the shaft rear end, and a plurality of radially projecting vanes pivotal on the hub. An axially shiftable core shaft extending coaxially in the tube shaft has a rear end rearward of the tube-shaft rear end and a front end forward of the tube-shaft front end. Mechanism connecting the core-shaft rear end with the vanes pivots the vanes on the hub on axial displacement of the core shaft. An axial-thrust bearing on the core-shaft front end is connected via another coaxial shaft to a piston having front and rear faces exposed in respective front and rear compartments formed by a cylinder fixed in the housing and surrounding the piston. Thus pressurization of the front compartment shifts the core shaft rearward and pressurization of the rear compartment shifts the core shaft forward.
The problem with this construction is that it is axially quite long. This makes it impossible to incorporate in an outboard drive, and even fairly complex in inboard or other drive types.